Combustion and Flame Class 8 Extra Questions and Answers

These AP 8th Class Physics Important Questions and 10th Lesson Combustion and Flame Class 8 Extra Questions will help students prepare well for the exams.

Class 8 Science Combustion and Flame Extra Questions

Combustion and Flame Class 8 Important Questions

Question 1.
Can you name a few fuels used in our homes ?
Answer:
LPG, kerosene, coconut oil, wood, cow dung cakes, coal etc.

Question 2.
Name a few fuels used in trade and industry.
Answer:
Petrol, diesel, LPG, CNG, Naphtha etc.,

Question 3.
What fuels are used for running automobiles ?
Answer:
Petrol, diesel, LPG, CNG etc.

Question 4.
What is the difference between the burning of a candle and the burning of a fuel like coal ?
Answer:
Candle burns with a flame whereas coal does not.

Question 5.
What products will be form when magnesium burns ?
Answer:
When magnesium burns, it forms magnesium oxide and produces heat and light.

Question 6.
Hold the piece of charcoal with a pair of tongs and bring it near the flame of a candle or a Bunsen burner. What do you observe ?
Answer:
We find that charcoal burns in air. We know that coal burns in air producing carbon dioxide, heat and light.

Question 7.
What is combustion ?
Answer:
A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

Question 8.
Can we say food) is a fuel for our body ?
Answer:
In our body food is broken down by reaction with oxygen and heat is produced. So, we can say that food is fuel to our body.

Question 9.
The sun produces its own heat and light. Is it also some kind of combustion ?
Answer:
In the sun, heat and light are produced by nuclear reactions but not combustion.

Question 10.
You might have heard that when the clothes of a person catch Are, the person is covered with a blanket to extinguish fire. Can you guess why ?
Answer:
A blanket wrapped around a person prevents the supply of air and stops the process of combustion.

Question 11.
What is ignition temperature ?
Answer:
The lowest temperature at which a substance catches fire is called its ignition temperature.

Question 12.
Can you tell now why a matchstick does not catch fire on its own at room temperature?
Answer:
The ignition temperature of the matchstick is more than that of the room temperature. So, a matchstick does not catch fire on its own at room temperature.

Question 13.
Why does the matchstick start burning on rubbing it on the side of the matchbox ?
Answer:
The temperature of matchsticks rises and reaches the ignition temperature and catches fire on rubbing it on the side of the matchbox.

Question 14.
What chemicals are present in the matchstick head ?
Answer:
A mixture of antimony trisulphide, potassium chlorate and white phosphorus with some glue and starch are present in the matchstick head.

Question 15.
Which substances are present in the rubbing surface of a matchbox ?
Answer:
The rubbing surface has powdered glass and a little red phosphorus.

Question 16.
Which substance is proved to be dangerous in the matchstick industry ?
Answer:
White phosphorus proved to,be dangerous both for the workers involved in the manufacturing of matches and for the users.

Question 17.
Why do all the combustible substances cannot catch fire at room temperature ?
Answer:
We find that a combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.

Question 18.
Have you ever seen cooking oil catching fire when a frying pan is kept for long on a burning stove ? Why ?
Answer:
Oil reaches its ignition temperature when kept for long on a burning stove and catches fire.

Question 19.
How can be fire controlled ?
Answer:
Fire can be controlled by removing one or more of the following

1. fuel
2. air supply
3. heat requirements.

Question 20.
What is the basic job of a fire extinguisher ?
Answer:
The job of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel, or both.

Question 21.
What is spontaneous combustion ?
Answer:
The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.

Question 22.
What is rapid combustion ?
Answer:
The combustion in which the fuel burns rapidly and produces heat and light is known as rapid combustion.

Question 23.
What will you do if a fire breaks out in your house or in your neighbourhood ?
Answer:
If a fire breaks out in your house or in your neighbourhood, the first thing to do is to call the fire service.

Question 24.
Why we should not pore water if electrical equipment is on fire ?
Answer:
If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire.

Question 25.
How is C02 stored in fire extinguisher ?
Answer:
CO₂ is stored at high pressure as a liquid in cylinders.

Question 26.
What are the different zones of candle flame ?
Answer:
There are three different zones of a flame – dark zone, luminous zone and non-luminous zone.

Question 27.
Which substances give heat energy for domestic and industrial purposes ?
Answer:
Heat energy for domestic and industrial purposes are mainly wood, charcoal, petrol, kerosene etc. These substances are called fuels.

Question 28.
Does an ideal fuel exist ?
Answer:

1. There is probably no fuel that could be considered as an ideal fuel.
2. We should look for a fuel which fulfill most of the requirements for a particular use.

Question 29.
What is caused by global warming ?
Answer:
Increased concentration of carbon dioxide in the air is believed to cause global warming.

Question 30.
Burning of coal and diesel releases sulphur dioxide gas. How is it harmful ?
Answer:
Sulphur dioxide gas is an extremely suffocating and corrosive gas.

Question 31.
Is it dangerous to sleep in a room containing a burning coal pot ?
Answer:

1. It is dangerous to burn coal in a closed room.
2. The carbon monoxide gas produced can kill persons sleeping in that room.

Question 32.
How is sulphur dioxide gas entered into the atmosphere ?
Answer:
Burning of coal and diesel releases sulphur dioxide gas into the atmosphere.

Question 33.
Write two chemicals which can produce CO₂ gas.
Answer:
Sodium bicarbonate (baking soda) and potassium bicarbonate.

Combustion and Flame Extra Questions

Question 1.
Write some combustible and non-combustible materials.
Answer:
Combustible substances : Wood, paper, kerosene oil, charcoal, wax, etc.
Non-combustible substances : Iron nail, stones, sand, soil, copper, etc.

Question 2.
What is combustible ? Give examples.
Answer:
The substance that undergoes combustion is said to be combustible. It is also called a fuel. Magnesium and Charcoal are combustible substances.

Question 3.
Why do you have to use paper or kerosene oil to start a fire in wood or coal?
Answer:

1. Substances catch fire when they reach their ignition temperature.
2. Paper and kerosene have low ignition temperatures so they can easily catch fire.
3. Coal and wood have high ignition temperatures. So, they do not catch fire easily.
4. So, we have to use paper or kerosene oil to start fire in wood or coal.

Question 4.
How do forest fires occur?
Answer:

1. During the extreme heat of summer, in some places dry grass catches fire.
2. From the grass, it spreads to trees and very soon the whole forest is on fire.
3. It is very difficult to control such fires.

Question 5.
Different substances catch fire at different temperatures. Why?
Answer:
Different substances have different ignition temperatures. So, different substances Cjatch fire at different temperatures.

Question 6.
What are inflammable substances ? Give Examples.
Answer:

1. The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
2. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG) etc.

Question 7.
How do fire brigades control the fires ?
Answer:

1. Fire brigades or fire fighters pour water on the fire.
2. Water cools the combustible material so that its temperature is brought below its ignition temperature.
3. This prevents the fire from spreading.
4. Water vapours also surround the combustible material, helping in cutting off the supply of air.
5. So, the fire is extinguished.

Question 8.
You have learnt that there are three essential requirements for producing fire. Can you list these requirements ?
(OR)
What are the three essential requirements for producing fire ? How ?
Answer:

1. The three essential requirements for producing fire are fuel, air and heat.
2. Fuel: Burns itself
3. Air: To supply oxygen
4. Heat: To raise the temperature of the fuel beyond the ignition temperature.

Question 9.
What is the common fire extinguisher ? When does it use ?
Answer:

1. He most common fire extinguisher is water.
2. But water works only when things like wood and paper are on fire.
3. It should not use when fire due to the electrical equipment.
4. Water is also not suitable for fires involving oil and petrol.

Question 10.
Water is also not suitable for fires involving oil and petrol. Why ?
Answer:

1. We know that water is heavier than oil.
2. So, it sinks below the oil and oil keeps burning on the top.
3. So, water is not suitable for fires involving oil and petrol.

Question 11.
Which fire extinguisher is best for fires involving electrical equipment and inflammable materials and electrical equipment ?
Answer:

1. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO₂) is the best extinguisher.
2. CO₂, being heavier than oxygen, covers the fire like a blanket.
3. Since the contact between the fuel and oxygen is cut off, the fire is controlled.
4. The added advantage of CO₂ is that in most cases it does not harm the electrical equipment.

Question 12.
How is CO₂ is best fire extinguisher ?
Answer:

1. CO₂ can be stored at high pressure as a liquid in cylinders.
2. When released from the cylinder, CO₂expands enormously in volume and cools down.
3. So, it not only forms a blanket around the fire, it also brings down the temperature of the fuel.
4. That is why it is an excellent fire extinguisher.

Question 13.
How do you get CO₂ gas if there is no availability of CO₂ fire extinguisher ?
Answer:
Another way to get CO₂ is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these chemicals give off CO₂.

Question 14.
Write causes of forest fires. What is our role to prevent it ?
Answer:

1. Spontaneous forest fires are sometimes due to the heat of the sun or due to lightning strike.
2. However, most forest fires are due to the carelessness of human beings.
3. It is important to remember that the camp fires must be completely extinguished before leaving a forest after a picnic, or a visit.

Question 15.
Give an example of an explosion.
(OR)
Explain the explosion with an example.
Answer:

1. Fireworks on festival days is an example of an explosion.
2. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound.
3. A large amount of gas formed in the reaction is liberated.
4. Such a reaction is called explosion.

Question 16.
How does a flame form ?
(OR)
Why does a candle burn with flame but not charcoal ?
Answer:

1. The substances which vapourise during burning, give flames.
2. For example, kerosene oil and molten wax rise through the wick and are vapourised during burning and form flames.
3. Charcoal, on the other hand, does not vapourise and so does not produce a flame.

Question 17.
Write the colours of a flame at different zones.
Answer:
Different zones of candle flame and its colours :

1. Luminous zone : Outer zone of complete combustion (blue)
2. Non-luminous zone : Middle zone of partial combustion (yellow)
3. Dark zone : Innermost zone of unburnt wax vapours (black)

Question 18.
Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. Why do they use the outermost zone of the flame ?
Answer:

1. The outermost zone of the flame is very hot due to complete burning of the fuel than that of inner and middle zones.
2. To get maximum heat, Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver.

Question 19.
What is a good fuel ?
(OR)
Which properties make a fuel a good fuel ?
Answer:

1. A good fuel is one which is readily available.
2. It is cheap.
3. It burns easily in air at a moderate rate.
4. It produces a large amount of heat.
5. It does not leave behind any undesirable substances.

Question 20.
What is calorific value ? Write its units.
Answer:

1. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
2. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question 21.
What is acid rain? How is it harmful ?
Answer:

1. Oxides of sulphur and nitrogen dissolve in rain water and form acids.
2. Such rain is called acid rain.
3. It is very harmful for crops, buildings and soil.

Question 22.
Which gas will be released in an incomplete combustion? How is it dangerous ?
Answer:

1. Incomplete combustion of these fuels gives carbon monoxide gas.
2. It is a very poisonous gas.
3. It is dangerous to burn coal in a closed room.
4. The carbon monoxide gas produced can kill persons sleeping in that room.

Question 23.
Choosing of wood for the purpose of cooking is not advisable. Why ?
(OR)
How is burning of wood harmful?
Answer:

1. Burning of wood gives a lot of smoke which is very harmful for human beings.
2. It causes respiratory problem.
3. Also, trees provide us with useful substances which are lost when wood is used as fuel.
4. Moreover, cutting of trees leads to deforestation which is quite harmful to the ‘ environment.

Question 24.
What is Global warming? What are its effects ?
Answer:

1. Global warming is the rise in temperature of the atmosphere of the earth.
2. This results, among other things, in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coasted areas.
3. Low lying coastal areas may even be permanently submerged under water.

Question 25.
Can we replace diesel and petrol by any other harmless fuels ?
(OR)
We should use CNG instead of petrol and diesel. Why ?
Answer:

1. The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts.
2. CNG is a cleaner fuel.

Question 26.
How is consumption of fuel being harmful ?
Answer:
Combustion of fuels release fine dust particles, carbon monoxide,carbon dioxide, sulphur dioxide etc.

1. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
2. The carbon monoxide gas produced is very dangerous, can kill persons.
3. Increased concentration of carbon dioxide in the air is believed to cause global warming.
4. The sulphur dioxide gas is an extremely suffocating and corrosive gas.
5. The acid rain is very harmful for crops, buildings and soil.

Question 27.
Write the differences between rapid and spontaneous combustion.
Answer:

Rapid combustion	Spontaneous combustion
The type of combustion in which a material burns rapidly and produces heat and light is known as rapid combustion. It occurs intentionally. Ex : LPG	The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous qombustion. It occurs spontaneously. Ex: Phosphorus

Question 28.
Make a list of fuels familiar to you. Group them as solid, liquid and gaseous fuels as in the table.
Types of Fuels

S.No.	Solid Fuels	Liquid Fuels	Gaseous Fuels
1.	Coal	Kerosene Oil	Natural gas
2.
3.

Answer:

S.No.	Solid Fuels	Liquid Fuels	Gaseous Fuels
1.	Coal	Kerosene Oil	Natural gas
2.	Wood	Petrol	LPG
3.	Dung cakes	Diesel	Coal Gas

Question 29.
In an experiment1 2.25 kg of fuel was completely burnt. The heat produced was measured to be 90,000 KJ. Calculate the calorific value of the fuel.
Answer:
Calorific value = heat produced/amount of the fuel
= 90000/2.25 = 40000 kJ/kg

Extra Questions of Combustion and Flame Class 8

Question 1.
What is combustion ? How many types of combustions are there ? Explain.
Answer:

1. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called a fuel.
2. There are various types of combustions such as rapid combustion, spontaneous combustion, explosion, etc.
3. Rapid combustion : The type of combustion in which a material burns rapidly and produces heat and light is known as rapid combustion. Ex : LPG.
4. Spontaneous combustion : The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion. Ex : Phosphorous.
5. Explosion: A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Ex : Fire crackers

Question 2.
How do matchsticks work ?
Answer:

1. A mixture of antimony trisulphide, potassium chlorate and white phosphorus with some glue and starch was applied on the head of a match made of suitable wood.
2. When struck against a rough surface, white phosphorus got ignited due to the heat of friction.
3. This started the combustion of the match.
4. The rubbing surface has powdered glass and a little red phosphorus.
5. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus.
6. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

Question 3.
What happens when Kerosene oil and wood are kept in the room temperature and heated continuously?
(OR)
How do you say that the ignition temperature of kerosene oil is lower than that of wood?
(OR)
We need to take special care in storing kerosene oil. Why?
Answer:

1. Kerosene oil and wood do not catch fire on their own at room temperature.
2. But, if kerosene oil is heated a little, it will catch fire.
3. But if wood is heated a little, it would still not catch fire.
4. That means the ignition temperature of kerosene oil is lower than that of wood.
5. We need to take special care in storing kerosene oil.

Question 4.
Observe the given table and answer the following questions.
Calorific values of different fuels

Fuel	Calorific Value (kJ/kg)
Cow dung cake	6000 – 8000
Wood	17000 – 22000
Coal	25000 – 33000
Petrol	45000
Kerosene	45000
Diesel	45000
Methane	50000
CNG	50000
LPG	55000
Biogas	35000 – 40000
Hydrogen	150000

Questions:
1) What values are given here ?
2) What is the units of calorific value ?
3) Which fuel has highest calorific value ?
4) Which fuel is best among wood, coal, dung cake ?
5) Which fuels are available in liquid state ?
6) Which gaseous fuel gives low energy ?
Answer:
1) The calorific values of fuels are given.
2) kJ/kg.
3) Hydrogen fuel has highest calorific value.
4) Coal is best fuel among wood, coal, dung cake.
5) Petrol, kerosene and diesel are available in liquid state.
6) Bio gas fuel gives low energy.

Question 5.
Write the harmful effects of the increasing fuel consumption.
Answer:

• The increasing fuel consumption has harmful effects on the environment.
• Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
• Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping.in that room.
• Combustion of most fuels releases carbon dioxide in the environment. Increased concentration! of carbon dioxide in the air is believed to cause global warming.
• Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen.
• Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

Question 6.
Where do you find spontaneous combustion and rapid combustion in your daily life ?
Answer:
Spontaneous combustion is observed in burning of match stick. When the match stick is struck against the rubbing surface the red phosphorous converts into white phos¬phorous which immediately react with potassium chlorate in the match stick head to produce enough heat to ignite antimony sulphide and start the combustion.

Rapid combustion is observed in gas stoves. When we turn the nob of the gas stove in the kitchen and bringing a burning match stick near to it. The gas burns rapidly and produce heat and light.

Question 7.
In few years the fuel on earth will be exhausted. Predict the consequences.
Answer:
If people unable to use alternative sources of energy then lack of fuels drastically affect the human civilization. The consequences of lack of fuel.

1. There would be no fuel for cooking.
2. There would be no fuel for transport.
3. There would be no fuel for running machinery.
4. There would be no fuel for producing electricity.
   So life of people on earth is miserable. The people once again go back to stone age
   where there is no availability of any facilities.

Question 8.
Why does candle give flame when it is burnt but why does coal burn without emitting a flame ?
Answer:
1) A candle is made of wax in which a thick thread is inserted. When it is lighted melts by a match stick. A little of the wax forms gas. This gas combines with oxygen in the air to form flame.

2) But coal is a natural fuel contains more carbon. When it is burnt it sends out carbon monoxide and carbon dioxide gases and becomes ember and does not emit a flame. Coal is the changed material of wood. Hence it does not emit a flame as it possesses a huge calorific value (heat).

Question 9.

Fuel	Calorific Value (K.J / Kg)
Cow dung	6,000 – 8,000
Coal	25,000 – 30,000
Petrol, Diesel	45,000
LPG	55,000
Hydrogen	1,50,000

Answer the following questions.
i) Name the fuel having highest calorific value.
ii) How much heat energy is released when one kg of petrol burnt ?
iii) Name two fuels which causes less pollution.
iv) Mention any one of the alternate energy source which is not mentioned in the above table.
Answer:
i) Hydrogen
ii) 45,000 Kilo Joules
iii) Hydrogen, L.P.G
iv) Solar power, Gobar gas, Wind power, Bio-diesel.

Question 10.
Prove the given statements.
i) Unburnt fuel particles present in the luminous zone of a flame.
ii) The non-luminous zone of the flame has a high temperature.
Answer:

1. Light a candle.
2. When the candle flame is steady, introduce a clean glass plate/slide into the luminous zone of the flame.
3. Hold it there with a pair of tongs for about 10 seconds.
4. Then remove it.
5. You can observe that a circular blackish ring is formed on the glass plate/slide.
6. It indicates the deposition of unburnt carbon particles present in the luminous zone of the flame.
7. Hold a thin long copper wire just inside the non-luminous zone of flame for about 30 seconds.
8. Notice that the portion of the copper wire just outside the flame gets red hot.
9. It indicates that the non-luminous zone of the flame has a high temperature. In fact, this part of the flame is the hottest part.